This invention relates to electronic switches and particularly to circuitry for improving the turn-off response time of semiconductor bridge switches and minimizing signal feedthrough the bridge when switching from its forward to reverse bias state. The invention further relates to circuitry for symmetrically controlling the switching of the bridge semiconductors from their forward to reverse bias state and for rapidly discharging the effective electrical capacitance of the bridge to reduce its turn-off time when it is switched from its forward to reverse bias state.
Electronic switches utilizing diode and transistor bridges are extensively used in communication and data systems to provide for high speed switching of information. For example, such switches are used in time division switching system applications for establishing recurrent, short duration connections for telephone call processing, conversation and supervision. Among the advantages of the switches are their low power consumption, fast acting time and small size compared to electro-mechanical switches.
Semiconductor bridges are customarily equipped with input and output signaling nodes and a pair of bias control nodes. Information signal transmission occurs between the input and output nodes and in response to the forward biasing of the bridge semiconductors by currents supplied to the biasing control nodes. In the absence of the forward bias, the bridge semiconductors substantially block signal transmission between the input and output nodes.
A problem in prior art semiconductor bridge arrangements is that the switching response time of the bridges is impaired due to residual charges stored on the effective electrical capacitance of the bridge following a withdrawal of the forward bias currents from its bias control nodes. The residual charge is generally dependent on the magnitude of the information signal switched through the bridge. Unfortunately, such a charge is usually dissipated, or discharged, via the signaling path including the output node. As a consequence, the discharge tends to increase the turn-off time of the bridge and, furthermore, tends, for example, to be a source of crosstalk between adjacent time slot channels sharing a common communication bus in a time division system.
In view of the foregoing, it is apparent that a need exists for facilities which minimize the deficiencies including crosstalk and impaired switching time resulting from the effective electrical capacitance of semiconductor bridges.